The inkjet printing method records images or characters by sticking fine droplets of ink compositions onto the substrates to be printed, and is characterized by the fact that it does not use a plate in the printing process. Another well-known printing method not using a plate is the electrophotographic printing method, but the inkjet printing method is considered superior in such aspects as device cost, running cost and printing speed, and the demand for inkjet printing is further expanding as there is increased demand for on-demand printing.
Concurrently with the improvement of the performance of the inkjet heads in the recent years, a focus is being placed on how to deal with small-lot printing in the existing printing market. In the printing market, productivity is important, and the single-pass mode characterized by its high printing speed is mostly used. A wide variety of printing substrates exist in the printing market, such as paper-based substrates and film-based substrates, and the inks may show different levels of adhesion depending on the substrates. In order to deal with high printing speed and multi-substrate printing, active energy ray-curable inks having fast cure speed and excellent versatility with respect to substrate adhesion are most suitable.
In the single-pass mode, missing dots (also called missed nozzles) caused by clogging of the nozzle etc. would conspicuously affect the image quality, and therefore the inks are required to have good jettability. Therefore, it is desirable that a cleaning liquid have not only excellent cleaning ability but also ability to suppress pigment aggregation which may occur upon the ink cleaning and cause jetting failure (i.e. ability to stabilize dispersion of the pigments), as well as preserving capability, e.g. ability to preserve water repellency of the nozzle plate.
However, the currently existing cleaning and preserving liquids (hereinafter also referred to as maintenance liquids), when removing the ink, would dilute the ink concentration to disturb the dispersion of the pigments, which likely leads to the clogging of the nozzle because of the aggregated pigments and to the eventual occurrence of jetting failure. Especially, when the ink is cleaned off and then the head is filled with a maintenance liquid to ship the printer, the ink will be kept in a dilute condition for an extended period of time, and the risk of clogging the nozzle will increase significantly. Moreover, the pigment surface following the disturbance of the dispersion is in a state of very high surface free energy, which causes the pigment to adhere not only to the printer and the ink passages within the head, but also strongly to the nozzle plate surface, causing deterioration of the water-repellency of the nozzle plate.
Thus, in developing a maintenance liquid for active energy ray-curable inkjet inks, it is important to impart good cleaning ability and preserving ability to the liquid.
Previously considered cleaning liquids include those of Patent Documents 1 and 2, which use a surface control agent to achieve an excellent cleaning ability and jettability, but these liquids are not designed for preserving ability, and therefore do not support long-term storage where the ink is left in a dilute condition. Moreover, these liquids are cleaning liquids for aqueous inks and are not suited for use with active energy ray-curable inks. On the other hand, the cleaning liquid for active energy ray-curable inks according to Patent Document 3 comprises a dispersant resin to achieve cleaning ability as well as dispersion stability of the pigments included in the ink. However, the pigment dispersant resins used with the active energy ray-curable inks are of high adsorbability, which is unlike the electric repulsion-based dispersion used for example in aqueous settings. Therefore, if there are excess dispersant resins in the maintenance liquid that do not adsorb to the pigments, the dispersant resins would adsorb to the nozzle plate surface, impairing the water repellency and substantially reducing the jetting stability.
As outlined above, cleaning liquids for removing active energy ray-curable inkjet inks and preserving liquids having such qualities as excellent cleaning ability and preserving ability have not been available in the current state of art.